Fields of Disclosure
The disclosure relates generally to the field of fluid separation. More specifically, the disclosure relates to a method and system of controlling a temperature within a melt tray assembly of a distillation tower.
Description of Related Art
This section is intended to introduce various aspects of the art, which may be associated with the present disclosure. This discussion is intended to provide a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The production of natural gas hydrocarbons, such as methane and ethane, from a reservoir oftentimes carries with it the incidental production of non-hydrocarbon gases. Such gases include contaminants, such as at least one of carbon dioxide (“CO2”), hydrogen sulfide (“H2S”), carbonyl sulfide, carbon disulfide, and various mercaptans. When a stream being produced from a reservoir includes these contaminants mixed with hydrocarbons, the stream is oftentimes referred to as “sour gas.”
Many natural gas reservoirs have relatively low percentages of hydrocarbons and relatively high percentages of contaminants. Contaminants may act as a diluent and lower the heat content of the produced hydrocarbon stream. Additionally, in the presence of liquid water, some contaminants can become corrosive to carbon steel.
It is desirable to remove contaminants from a stream containing hydrocarbons to produce sweet and concentrated hydrocarbons. Specifications for pipeline quality natural gas typically call for a maximum of 2-4% CO2 and ¼ grain H2S per 100 standard cubic feet (scf) (i.e., 4 parts per million volume (ppmv)) or 5 milligrams per Normal meter cubed (mg/Nm3) H2S). Specifications for lower temperature processes such as natural gas liquefaction plants or nitrogen rejection units typically require less than 50 parts per million (ppm) CO2.
The separation of contaminants from hydrocarbons is difficult and consequently significant work has been applied to the development of hydrocarbon/contaminant separation methods. These methods can be placed into three general classes: absorption by solvents (physical, chemical and hybrids), adsorption by solids, and distillation.
Separation by distillation of some gas mixtures can be relatively simple and, as such, is widely used in the natural gas industry. However, distillation of mixtures of natural gas hydrocarbons, primarily methane, and one of the most common contaminants in natural gas, carbon dioxide, can present significant difficulties. Conventional distillation principles and conventional distillation equipment are predicated on the presence of only vapor and liquid phases throughout the distillation tower. The separation of CO2 from methane by distillation typically involves temperature and pressure conditions that result in solidification of CO2 if a pipeline or better quality hydrocarbon product is desired. The required temperatures are cold temperatures typically referred to as cryogenic temperatures.
Certain cryogenic distillations can overcome the above mentioned difficulties. These cryogenic distillations provide the appropriate mechanism to handle the formation and subsequent melting of solids during the separation of solid-forming contaminants from hydrocarbons. For example, the formation of solid contaminants in equilibrium with vapor-liquid mixtures of hydrocarbons and contaminants at particular conditions of temperature and pressure may take place in a controlled freeze zone section of a distillation tower.
The controlled freeze zone section typically comprises a melt tray assembly that collects and warms solids that form in the controlled freeze zone section. Liquid in the melt tray assembly helps conduct heat to warm the solids and create a liquid slurry. The melt tray assembly provides adequate heat transfer to melt the solids and facilitate liquid slurry draw-off to a stripper section of the distillation tower.
Maintaining the liquid in the melt tray assembly at a steady-state conditions is important for overall process stability within the distillation tower. Too high of a temperature can result in decreased separation performance of the contaminants from the stream containing the hydrocarbons in the controlled freeze zone section, which in turn can result in higher contaminant content in the stream flowing through a rectifier section of the distillation tower and/or can lead to solid formation in the rectifier section. Solid formation in the rectifier section can cause a disruption within the distillation process and prevent adequate removal of the contaminants from the stream. Conversely, too low of a temperature can result in solid formation in the melt tray assembly, which can stop flow of the liquid slurry into the stripper section, thereby disrupting operation within the distillation process. Therefore, a melt tray assembly with unsteady temperatures can negatively affect the rate of removal of contaminants from the stream in the controlled freeze zone section, which may detrimentally affect the purity of the recovered hydrocarbons and may increase the operational costs of the distillation process.
Maintaining the distillation tower in a distillation process in a way such that there is not a sudden change in melt tray assembly duty requirements is important for overall process stability of the distillation tower. A sudden change in melt tray assembly duty requirements may occur when abnormal operation occurs in the distillation process such as, but not limited to, when there is an upset in a reboiler within the distillation process, unexpected solid accumulation within the distillation tower and/or an upset in spray rate within the controlled freeze zone section from a spray assembly. When the sudden change occurs, the distillation tower must be modified to return to normal operation.
Conventional controlled freeze zone sections comprise a melt tray assembly with a melt tray heat exchange device having a single-phase heat transfer fluid. The melt tray heat exchange device is used to facilitate the warming of the solids, formed by a spray assembly in the controlled freeze zone section, in the melt tray assembly. However, disadvantages can result when using a single-phase heat transfer fluid. For example, the single-phase heat transfer fluid can cause a temperature differential between an inlet and an outlet of the melt tray assembly, which can make it difficult to hold a steady temperature within the melt tray assembly. Further, sudden changes in the melt tray assembly duty requirements can result in rapid changes in the necessary single-phase heat transfer fluid flow rate needed, thereby making it difficult for the desired temperature to be maintained in the melt tray assembly.
A need exists for improved technology that can better facilitate heat transfer within the melt tray assembly, can maintain steady-state conditions within the melt tray assembly, does not generate a substantial temperature differential within the melt tray assembly, and/or can effectively manage rapidly changing heat duty requirements within the controlled freeze zone section.